Molecular memory with atomically smooth graphene contacts
© Umair et al.; licensee Springer. 2013
Received: 8 May 2013
Accepted: 13 October 2013
Published: 14 November 2013
We report the use of bilayer graphene as an atomically smooth contact for nanoscale devices. A two-terminal bucky-ball (C60) based molecular memory is fabricated with bilayer graphene as a contact on the polycrystalline nickel electrode. Graphene provides an atomically smooth covering over an otherwise rough metal surface. The use of graphene additionally prohibits the electromigration of nickel into the C60 layer. The devices exhibit a low-resistance state in the first sweep cycle and irreversibly switch to a high-resistance state at 0.8 to 1.2 V bias. In the subsequent cycles, the devices retain the high-resistance state, thus making it write-once read-many memory.
Reliable and efficient contacts are an important aspect of device design at the nanoscale level. Historically, the contacts in the micron-scale devices have only been part of the overall device design for minimizing the contact resistance based on Schottky barrier height [1–3]. At the nanoscale level, however, the influence of contacts on the transport channel is so important that an equal or often times even more effort is spent on the contact and interface design [4, 5]. In various nanoscale devices, the contacts even dominate the transport characteristics [6, 7]. While various novel contacts exist at the nanoscale with unique density of states, the simplest ones are the ohmic contacts used to inject and extract the charge carriers. However, in addition to the atomic roughness and grain boundaries, such contacts suffer from electromigration or filament formation, which may deteriorate the device characteristics and lead to reliability issues . One of the grand challenges thus for the nanoscale design is to provide smooth and reliable contact to nanomaterials, being free from electromigration and any other non-ideal effects. In this paper, our objective is to explore graphene [9, 10] nanomembranes as a candidate for such contacts. The use of graphene and boron nitride has been explored earlier for ultrathin circuitry .
In this work, we report the use of bilayer graphene (BLG) as an atomically smooth contact in a molecular memory. Although various device structures based on graphene have been explored , our study is unique in the context of its use to improve reliability. BLG may prevent the electromigration of Ni atoms into the active material of the device. Furthermore, the use of BLG instead of monolayer or several-layer graphene is twofold. As compared to the monolayer, the probability of complete coverage with BLG is higher in the presence of defects. On the other hand, with the increasing number of layers, the transport properties of the device may be dominated by the multilayer graphene itself. Thus, BLG tends to provide an optimum trade-off.
A detailed characterization of the synthesized BLG has been reported earlier in . Raman spectroscopy was used to confirm the quality of evaporated C60. A laser power of 2 mW with 5 s scan time and four scans per point is used to avoid sample heating. The Raman spectrum of evaporated C60 film on BLG is also shown in Figure 1c. The dominant peaks are at 491, 1,464, and 1,596 cm−1 wavenumbers, which confirm the coherence of C60 molecular structure even after thermal evaporation [14, 15].
Results and discussion
The switching behavior for the second sweep cycle is shown in Figure 2b. The device remains in the high-resistance state without hysteresis. In the subsequent sweep cycles, the device sustains its high-resistance state, thus making it a write-once read-many (WORM) memory device.
For the control sample without the BLG contact, the device shows higher conduction with random switching, hysteresis, and significant variation from device to device. We attribute this irregular behavior in our control sample to the atomically rough interface between Ni and C60, as well as the electromigration of Ni atoms across C60/Ni interface.
The switching mechanism in the reported WORM memory device with the BLG contact is not clearly understood yet. However, we hypothesize that BLG prevents the electromigration of Ni atoms into C60 film, thus stabilizing the device behavior. The transport characteristics do not show ohmic or space-charge-limited conduction. Similar devices using C60 molecules have been reported to have rewritable switching characteristics - quite different from our observation [19, 20]. Moreover, multilayer graphene electrodes used in devices with PI:PCBM composite as active material have also been recently reported to have WORM memory behavior, whereas with the metallic electrodes, rewritable switching characteristics have been reported . Although the channel materials are different in the two experiments, the connection between the use of graphene and WORM features is noteworthy and needs to be explored further. Carbon nanotube-based contact  has also been explored to eliminate electromigration, however, we believe that graphene nanomembrane provides a better interface due to its 2D nature.
We have fabricated a molecular memory device with atomically smooth BLG contacts. Covering Ni film with BLG shields the channel from metal surface irregularities and also prevents the electromigration of Ni atoms into the C60 film. The device switches from a low-resistance to a high-resistance state, followed by hysteresis in the first sweep cycle. In the subsequent sweep cycles, the device remains in the high-resistance state and no hysteresis is observed, thus showing WORM memory behavior. The switching voltages vary in 0.8 to 1.2 V bias range for various devices with the high-resistance to low-resistance ratio in 20 to 40 range. The retention characteristics show good endurance under both low-resistance and high-resistance states up to 104 s. In addition, replacing the top Cr/SiO2 contact with BLG may further improve the characteristics, which we leave for future work.
AU received his B.Sc. degree in Electrical Engineering from the University of Engineering and Technology, Lahore, Pakistan, in 2007 and is currently working towards his Ph.D. degree in Electrical and Computer Engineering at the University of Iowa. His research interests include novel non-volatile memories, resistive random access memories, flash memories, and carbon nanomaterial synthesis.
TR received her B.Sc. honors in May 2001 from the University of Engineering and Technology Lahore, Pakistan majoring in electronics and communication engineering. Afterwards, she worked in Accelerated Technologies Inc. Pakistan, as a software engineer. She worked in SIEMENS Pakistan, for another year before she joined Purdue University, West Lafayette, IN, USA for Ph.D. program. She graduated from her Ph.D. in December 2010 and joined the University of Iowa, USA as adjunct Assistant Professor in the Department of Electrical and Computer Engineering and Department of Physics and Astronomy. Presently, she is an Assistant Professor at Lahore University of Management Sciences, Pakistan.
HR is a Professor of Electrical Engineering at the University of the Punjab, Lahore, Pakistan since 2012. Earlier, he was an Assistant Professor of Electrical and Computer Engineering at the University of Iowa, Iowa City, USA in 2009 to 2013. He was a postdoctoral associate at Cornell University in 2007 to 2009. He received his Ph.D. in 2007 and MS in 2002 from Purdue University; and B.Sc. in 2001 from the University of Engineering and Technology Lahore Pakistan. He has received ‘Magoon Award for Excellence in Teaching’ from Purdue University in 2004. He is also the recipient of ‘Presidential Faculty Fellowship’ in 2010 and ‘Old Gold Fellowship’ in 2011 from the University of Iowa. He has been awarded ‘Junior Associateship’ of the International Centre for Theoretical Physics, Trieste, Italy in 2013. His research group is focused on ‘anything that is small’ for low-power post-CMOS transistor, spintronics, sensors, and solid-state energy harvesting applications from theoretical, experimental, and computational approaches using graphene, molecule, silicon, novel dielectrics, and carbon nanotube material systems. He has served as an editor of a 600-page book on Graphene Nanoelectronics published by Springer in 2012.
We thank D. Norton, C. Coretsopoulos, and J. Baltrusaitis for useful discussions. We acknowledge the Microfabrication Facility at the University of Iowa for evaporation, and Central Microscopy Research Facility at the University of Iowa for Raman spectroscopy. This work is supported by the MPSFP program of the VPR office at the University of Iowa.
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